Lecture 3

• Microbial Communities

  Microbes naturally exist in communities, interacting in complex ways that affect their growth, behaviour, and survival
• Growth and Interactions in Microbial Communities

  1. Microbes exhibit exponential growth, leading to high cell densities where interactions become crucial

  2. Mixed cultures feature different growth rates and needs, resulting in varied outcomes and abundances
• Quorum Sensing

  A cell-density dependent mechanism by which bacteria communicate and coordinate their behavior through chemical signals
• Autoinducers

  Bacteria release molecules called autoinducers into their environment, which increase in concentration as the bacterial population grows
• Detection and Response in Quorum Sensing

  1. Bacteria detect the concentration of autoinducers to estimate the density of their population

  2. When the autoinducer concentration reaches a threshold level, it signals the bacterial cells to alter gene expression collectively, initiating a coordinated response
• Types of Autoinducers

  • Acyl-homoserine Lactones (AHLs): Used by many Gram-negative bacteria
  • Autoinducing Peptides: Utilized by Gram-positive bacteria, often with a thiolactone ring structure
• Agr System in Staphylococcus aureus

  • AgrD: Precursor of the autoinducing peptide
  • AgrB: Processes and secretes the mature autoinducing peptide
  • AgrC: The receptor that binds the autoinducing peptide and activates the response regulator AgrA
  • AgrA: When phosphorylated, it activates the P3 promoter, which leads to the production of RNAIII
• Low vs. High Cell Density in Staphylococcus aureus

  • At low cell density, little AgrD is produced, facilitating adhesion and biofilm formation
  • As cell density increases, the Agr system triggers a switch to a more invasive phenotype by expressing virulence factors and repressing adhesion
• RNAIII

  • A regulatory RNA in Staphylococcus aureus that controls many genes, including those for adhesion and virulence factors
  • It can directly regulate the expression of toxins, like α-hemolysin
  • RNAIII forms complex secondary and tertiary structures that can bind to and activate the translation of specific mRNA molecules
• Activation of the Agr system and RNAIII production in Staphylococcus aureus

  Pivotal for the bacteria's transition from a commensal organism to an aggressive pathogen
• Biofilms

  Structured communities of microbial cells that are enclosed in a self-produced polymeric matrix and adhere to biological or non-biological surfaces
• Biofilm Composition

  The matrix is composed of various substances, including exopolysaccharides, proteins, and nucleic acids
• An estimated 80% of all microbial biomass on Earth exists within biofilms
• Biofilms

  Enable microorganisms to survive in various environments and protect them from hostile conditions
• Biofilms and Associated Consequences

  • Contact Lenses: Pseudomonas aeruginosa, Staphylococcus epidermidis - Can lead to keratitis, an inflammatory condition of the cornea
  • Urinary Catheters: E. coli, Pseudomonas aeruginosa, Proteus mirabilis - Can result in bacteriuria, the presence of bacteria in the urine
  • Joint Replacements: Staphylococcus epidermidis, Staphylococcus aureus - Can lead to septicemia, a serious bloodstream infection, or device failure
• Biofilm Formation Stages

  1. Initial Attachment: Microbial cells utilize structures like flagella and type I pili to make initial contact with a surface

  2. Irreversible Attachment: Further adhesion is reinforced through lipopolysaccharides (LPS) and Type IV pili, establishing a more permanent connection

  3. Maturation I: Cells form microcolonies and produce alginate, which is part of the extracellular polymeric substance (EPS), and repress flagella as they are no longer needed for movement

  4. Maturation II: Quorum sensing mechanisms regulate further biofilm development

  5. Dispersion: The biofilm matures to a point where it releases cells back into the environment, continuing the cycle of colonization
• Biofilms

  • The biofilm matrix acts as a shield against the host immune response, desiccation, and biocides, including antibiotics and disinfectants
  • Biofilm formation is often initiated by extracellular signals present in the environment, signaling the bacteria to begin the biofilm formation process
• Multispecies Biofilms

  • Biofilms function like miniature ecosystems, with different microbial species performing various functions and interactions
  • Due to the often-lack of oxygen in biofilms, anaerobic metabolic processes are common
  • Different species within a biofilm can use the waste products of others, effectively recycling nutrients
• Genes in Biofilms

  • Pseudomonas aeruginosa: Twitching motility, linked to Type IV pili expression and alginate production, is controlled by the sigma factor σ^22, encoded by the algT gene in the alginate synthesis operon
  • Vibrio parahaemolyticus: Swarming motility, essential for biofilm maturation, involves a switch between two different flagella systems, which can be induced by environmental signals like surface contact that interferes with flagella rotation
• Quorum Sensing in Staphylococcus aureus

  S. aureus utilizes quorum sensing to regulate the transition from a commensal state to a virulent pathogen within a biofilm
• RNAIII in Staphylococcus aureus

  Acts as both a direct and indirect regulator of gene expression, controlling a network of genes within the biofilm community